Application of AC Foaming Agent in PVC Profiles

Adding some fillers to plastic can improve certain properties of the plastic, making it more suitable for certain specialized uses. To reduce the density and hardness of plastic, or to enhance its thermal or acoustic insulation, the ideal filler is voids. Plastics containing voids or foam-like structures are classified as foam plastics. Depending on the degree of foaming, that is, the volume fraction of foam caused by voids, the properties of foam plastics can be significantly different from the base plastic. AC foaming agent is a chemical that can be added to plastic, and at the appropriate time during the processing, it releases gas to form bubbles in the plastic. The formation of plastic foam generally can be divided into four stages.

What Is AC Foaming Agent

AC foaming agent is produced by the oxidation of urea, appearing as a light yellow or orange-yellow crystalline powder. The molecular weight of the AC foaming agent is 116, with a decomposition heat of 359.9 J/g°C. The gas released upon decomposition is mainly nitrogen (65%), carbon monoxide (32%), and a small amount of carbon dioxide (3%). The solid residues from the decomposition are primarily biuret, cyanuric acid, and urazole. It has a slight ammonia smell when decomposing, is non-flammable, and self-extinguishing. It is stable when stored at room temperature and can be considered non-toxic. Due to its excellent performance, the AC foaming agent is widely used, especially in rigid PVC foamed profiles.

The Four Stages of the Foaming Process

First Stage

The AC foaming agent must be completely and uniformly dispersed in the polymer, which is usually in a liquid or molten state. At this stage, the AC foaming agent can form a true solution in the polymer or merely be uniformly dispersed in the polymer, forming a biphasic system.

Second Stage

After forming a large number of individual bubbles, the system becomes one where gas is dispersed in liquid. At this point, nucleating agents are often added to promote the formation of numerous small bubbles. Nucleating agents are generally very fine inert particles that provide sites for the new gas phase to form.

Third Stage

The initially formed bubbles continuously expand as more gas diffuses and permeates through the polymer into the bubbles. If this process continues long enough, individual bubbles will start to contact each other. If the walls separating individual bubbles rupture, larger bubbles will form through this coalescence. If the foam is primarily formed by interconnected bubbles, it is called open-cell foam. If the foam is formed by unconnected bubbles, it is called closed-cell foam. If bubble coalescence is allowed to continue unrestrictedly, the foam will collapse as the gas completely separates from the polymer.

Fourth Stage

When the polymer viscosity increases and bubbles can no longer grow, the foam stabilizes. Increasing the polymer viscosity can be achieved through cooling, cross-linking, or other methods.